I might have just obtained a Washburn XB-100 from a friend who defretted it himself. I was wondering what type of filling or putty I should use to fill in the cracks. Also I was toying with the idea of installing a J pup. As of right now the bass has a single p pup almost dead center between the bridge and the fingerboard. I have the tools and ability to route out a slot for the p pup closer to the bridge and install a j at the bridge but what tactics are there to hiding the old split p pup cavity? (spelling)
You could glue in wood veneer or sheet plastic (get it at a model store) to fill the slots. Probably a better solution than wood putty.
Does the bass have a pickguard? If so, you could possibly make a new pickguard from blank pickguard material to cover the old pickup cavity. Other than that, you'd have to cut matching wood plugs to fill the cavity, but this would involve refinishing the bass. You could also plane down the top about 1/4" and then put on a figured wood top and route it all new however you wanted, but again, filling the cavity and retopping a guitar is a considerable undertaking. Why the desire to move the pickup closer to the bridge? Why not just route a hole for a j-style or soapbar pickup close to the bridge and add it while leaving the original in there. You could go with several different wiring options, a blend pot, or a jazz bass style with two volumes and a tone, or maybe even a Les Paul style selector switch, etc. :^)~